An R-T-B based sintered magnet (where R is at least one of rare earth elements, indispensably containing Nd, and T is a transition metal element, indispensably containing Fe) is composed of a main phase made of a compound having an R2T14B type crystal structure and a grain boundary phase located at a grain boundary portion of this main phase, which is known as a magnet with the highest performance among permanent magnets.
Therefore, this type of magnet is used in various applications such as voice coil motors (VCM) of hard disk drives, motors for electric automobile (EV, HV, PHV), and motors for industrial equipment, and home appliance.
With the expansion of applications, the motor for electric automobile is sometimes exposed to high temperature in a range of 100° C. to 160° C., thus requiring a stable operation even at high temperature.
However, the R-T-B based sintered magnet has its coercive force HcJ (hereinafter simply referred to as “HcJ” in some cases) reduced at high temperatures, leading to irreversible thermal demagnetization. When the R-T-B based sintered magnet is used in motors for electric automobile, use of the R-T-B based sintered magnet at high temperature leads to a reduction in HcJ, thus failing to obtain a stable operation of the motor. Therefore, there is required an R-T-B based sintered magnet which has high HcJ at room temperature and also high HcJ at high temperature.
Conventionally, to improve HcJ at room temperature, heavy rare earth elements (mainly Dy) have been added to the R-T-B based sintered magnet. However, this results in a problem that a residual magnetic flux density Br (hereinafter simply referred to as “Br” in some cases) is reduced. Dy has various issues, including inconsistent supply and large fluctuations in price due to restricted areas where their resources are located, and the like. For this reason, users request technology which enables an improvement in HcJ of R-T-B based sintered magnets without using heavy rare-earth elements RH, such as Dy, as much as possible.
Patent Document 1 discloses, as such technology, technology in which the B content is set lower than that in the standard R-T-B based alloy, while at least one element selected from Al, Ga, and Cu is contained as a metal element M to thereby form an R2T17 phase, thus ensuring an adequate volume ratio of a transition metal-rich phase (R6T13M) formed using the R2T17 phase as a raw material, whereby an R-T-B based rare-earth sintered magnet with high coercivity can be obtained while reducing the Dy content.